Composite materials such as oriented strand board, particle board and flake board are generally produced by blending or spraying comminuted lignocellulosic materials such as wood flakes, wood fibers, wood particles, wood wafers, strips or strands, pieces of wood or other comminuted lignocellulosic materials with a binder composition while the comminuted materials are blended by tumbling or agitating them in a blender or like apparatus. After blending sufficiently to form a uniform mixture, the materials are formed into a loose mat, which is compressed between heated platens or plates to set the binder and bond the flakes, strands, strips, pieces, etc., together in densified form. Conventional processes are generally carried out at temperatures of from about 120 to 225° C. in the presence of varying amounts of steam, generated by liberation of entrained moisture from the lignocellulosic materials. These processes also generally require that the moisture content of the lignocellulosic materials be between about 2 and about 20% by weight, before the lignocellulosic materials are blended with the binder composition.
Plywood production is accomplished by roll coating, knife coating, curtain coating, or spraying a binder composition onto veneer surfaces. In addition, foamline and extrusion processes can be used. A plurality of veneers are then laid-up to form sheets of required thickness. The mats or sheets are then placed in a heated press and compressed to effect consolidation and curing of the materials into a board. Binder compositions which have been used in making such composite wood products include phenol formaldehyde resins, and urea formaldehyde resins.
Isocyanates are commercially desirable for use in forming some types of composite materials because they have low water absorption, high adhesive and cohesive strength, flexibility in formulation, versatility with respect to cure temperature and rate, excellent structural properties, the ability to bond with lignocellulosic materials having high water contents, and produce no formaldehyde emissions. Moreover, the isocyanates absorb into some types of the composite materials to form a 3D matrix. The disadvantages of using isocyanates include difficulty in processing due to their high reactivity, adhesion to platens, lack of cold tack, high cost and the need for special storage.
However, relative specifically to forming plywood, isocyanates historically cannot be used because they fully absorb into the veneers and are not viscous enough to be used as adhesives, thereby leading to run-off and material loss during industrial processing. Moreover, even if isocyanates are tried for use as adhesives for plywood, the bond strength is insufficient to adhere the veneers because a 3D matrix is not formed. For these reasons, there remains an opportunity to develop and improved composition and process for forming plywood.